With the widespread popularity of various image-pickup devices such as digital cameras and cellular phones with cameras, demand for the glass lenses employed in these devices is increasing. In such devices, there is a need to pick, up high-quality images with a small number of aspherical lenses or small lenses. Glass precision press molding methods are employed to provide such lenses with high productivity.
In precision press molding, a premolded product called a preform is prepared from a quantity of high-quality glass equal to that of a press molded product. The preform is heated and pressed in a pressing mold to precisely transfer the molding surface of the pressing mold to the glass. The productivity of this method is affected by the productivity of the preform manufacturing step in addition to the productivity of the precision press molding step.
Generally, in preform manufacturing methods, a glass melt is caused to flow into a casting mold to form a glass block, after which the glass block is cut into glass pieces and the glass pieces are ground and polished in what is known as a cold processing method, or a glass melt gob the size of one preform is separated from a glass melt and the glass is molded into a preform in a cooling process in what is known as a hot molding method (see Japanese Unexamined Patent Publication (KOKAI) No. 2003-40632). Since preforms are directly manufactured from glass melt in hot molding methods, these methods afford better productivity than cold processing methods.
As stated above, hot molding methods are advantageous methods. However, they require greater technical ability than cold processing methods. For example, when molding preforms of ultralow dispersion fluorophosphate glass, the fluorine in the glass reacts with the atmosphere and vaporizes when the temperature of the glass is elevated during molding, sometimes producing striae in the surface of the preform. When a lens is molded from such a preform, an optically heterogeneous portion is produced in the surface of the lens, compromising the performance of the lens. In glass containing alkali metal components or B2O3, when elevated temperature is employed in the course of causing the glass melt to flow, volatization of the alkali metal or B2O3 component generates striae in the preform. It is necessary to employ various measures suited to the glass to reduce striae in order to resolve this problem of reduced preform productivity due to the generation of striae.
The present invention, devised to solve the above-stated problem, has for its object to provide methods and devices for manufacturing glass preforms with reduced striae by hot molding methods.
A further object of the present invention is to provide a method for manufacturing optical glass elements permitting the manufacturing of high-quality glass elements by precision press molding of the glass preforms with reduced striate obtained using the above devices and methods.